As is known by those skilled in the art, digital radio involves the transmission of a digital information signal from the transmitter, where it modulates a carrier signal, through a communications channel, and finally to a receiver. In the receiver, the received signal is demodulated to remove the carrier, the clock is recovered, and then the received signal is decoded to extract the original information signal. The resultant signal after demodulation is the familiar eye pattern. But because of the bandwidth and time constant characteristics of the transmitter, the channel, and the receiver, the demodulated signal lacks the sharp, clearly defined edges that the information signal had before passing through the transmitter. That is, the transmitter, channel, and receiver have inherent filter-like characteristics that cause the pulses in the demodulated signal to bleed together and overlap. As a result, the eye pattern is distorted. The key to effective decoding is to sharpen the eye pattern and to sample the demodulated signal when the eye has its widest opening. At this time there is minimal interference from later transmitted symbols and thus the likelihood of accurate decoding (i.e., determining the value of the transmitted information signal) is maximized.
The signal space constellations for 16 QAM, 64 QAM, and QPSK are shown in FIGS. 1A, 1B, and 1C, respectively. Each of these signal space constellations is formed by sampling a continuous time domain waveform at the appropriate time. The sampling time is chosen to be that time when only a single transmitted symbol (digital value or impulse) is present at the receiver and there is no intersymbol interference from earlier transmitted values. The superposition of many segments of the received analog waveform is referred to as an eye pattern, and it is this eye pattern that is used to determine when the sampling should occur. The most appropriate sample location on the eye pattern is that instant when the eye is open widest. FIGS. 2A, 2B, and 2C illustrate eye patterns for QPSK, 16 QAM, and 64 QAM, respectively modulation waveforms.
Returning to FIG. 1, it can be seen that the signal constellation space is in two dimensions, commonly referred to as the I and Q channels of the signal. These two channels are distinguishable from each other on the RF carrier because the channels are modulated on carriers that are orthogonal to each other and of the same frequency. Specifically, the I channel is often modulated on a carrier of the form cos .omega..sub.o t) and the Q channel is modulated on a carrier of the form sin.omega..sub.o t) . As is well known, these two mathematical functions are orthogonal to each other. This modulation scheme is well known and is commonly referred to as quadrature modulation. If the amplitude of the I/Q signals are also amplitude modulated, then the resultant is referred to as quadrature/amplitude modulation or QAM. The number of amplitude levels on the I and Q channel determines the type of QAM as illustrated in FIGS. 1A and 1B. Demodulation of the QAM and QPSK signals requires the demodulating oscillator to inject a heterodyning signal at the proper phase so that only one of the modulated channels is demodulated at a time. The other channel is demodulated in a similar manner by using a 90 degree phase shifted version of the first demodulating signal. This scheme is well known in the art and is illustrated in FIG. 3.
If the demodulating oscillator (the VCO local oscillator illustrated in FIG. 3) is phased properly with respect to the I and Q signals then the vector representation of the demodulated signals will be colinear with the I and Q axes as illustrated in FIG. 4A and the constellation space will be correctly situated as shown in FIG. 4B. FIG. 4C shows the vector representation when there is an oscillator phase error. Now the input signals, while still separated by 90 degrees, do not lie on the I and Q axis and the constellation space is skewed noticeably. FIG. 4E shows the resulting distortion in the I and the Q channels as determined by vector addition of the I and Q components from each of the input signals.
The prior art demodulator uses sweep-assisted acquisition on both the decision directed phase detector (which is a common type of phase detector suitable for use with 64 QAM modulation formats) and the clock phase detector in such a manner that the carrier and clock are attempting to acquire the signal at the same time. Since it is known that the clock must be acquired prior to the decision directed carrier phase detector acquiring, the usual method has been to sweep the carrier phase detector very slowly and the clock phase detector quickly, knowing that eventually the proper combination will occur and acquisition will be complete. While this method works, it is rather random in nature and therefore hard to predict exactly when the acquisition will be complete. In addition, the need to sweep the carrier phase detector slowly causes the process to take an excessive time.